1. Field of the Invention
The present invention relates to a disk-type recording medium which, while rotated, allows data to be recorded thereon and/or reproduced therefrom by a head, and also relates to a storage apparatus incorporating such a recording medium.
2. Description of the Related Art
A greater part of conventional hard-type magnetic recording media has been the type comprising a magnetic coating in the form of magnetic layers on either surface of a circular Al substrate. Such magnetic recording media have various outer diameters such as 14", 10.5", 9.5", 8", 5.25", 3.5" and 2.5". Their thickness is of the order of several mm, typically about 2 mm.
On the other hand, a magnetic recording apparatus has an arrangement for allowing several magnetic recording media to be stacked one above another with a single rotary shaft passing through the media so that a large area for writing magnetic recording information is assured, and the recorded information capacity of the apparatus is increased. Accordingly, in order that a magnetic recording apparatus may allow several magnetic recording media to be precisely stacked and easily mounted in said relationship with the rotary shaft, a magnetic recording medium includes a substrate having a through hole formed therein.
In, for example, U.S. Pat. No. 4,652,499 a magnetic recording medium is proposed which includes a substrate composed of an Si wafer of the same type as those for semiconductor devices. Also known are magnetic recording media in which glass, in particular, chemically strengthened glass, is used as the substrate material.
A disadvantage of the above-described construction resides in the fact that, when glass or an Si wafer is used as the material for forming the substrate of a recording medium, the strength of the medium may be greatly deteriorated by the formation of the through hole in the substrate. Specifically, cracks tend to be generated in the inner peripheral edge around the through hole when the substrate is being subjected to the through hole forming operation or when the recording medium is being fitted onto a spindle. During the rotation of the recording medium at a high speed, the cracks may grow, finally leading to the breakage of the recording medium. Thus, a problem arises in which the reliability of the recording medium with regard to its strength is low.
Another problem is that, in order to allow the strength of the recording medium to be maintained above a certain value, the thickness of the substrate cannot be reduced below a certain dimension.
A further problem is that, when each of the magnetic layers is being formed by certain vacuum thin layer forming art, such as a sputtering method, the existence of the through hole causes a part of the deposition particles ejected from the side of one of the substrate surfaces to be deposited on the other surface, thereby causing differences in the thickness of the formed layers between the vicinity of the through hole and the vicinity of the outer periphery of the substrate.